fix: 🐛 clarify error message in add()

Author: alanhussey

src/math/add.js

@@ -16,7 +16,10 @@ function add(...measurements) {
     return measurements[0];
   }
   if (!measurementsHaveSameUnit(measurements)) {
-    throw new TypeError('Cannot add measurements with different units');
+    const units = Array.from(new Set(measurements.map(m => m.unit.name)));
+    throw new TypeError(
+      `Cannot add measurements with different units: ${units.join(', ')}`
+    );
   }
   const unit = measurements[0].unit;
   return new Measurement(_add(measurements), unit);

src/math/add.test.js

@@ -26,7 +26,7 @@ describe(add, () => {
       new Measurement(1, foot),
     ];
     expect(() => add(...measurements)).toThrowError(
-      new TypeError('Cannot add measurements with different units')
+      new TypeError('Cannot add measurements with different units: inch, foot')
     );
   });